Touch-sensitive display devices typically include a screen for displaying data and an active layer arranged directly in front of the screen, between the observer and the screen. This active layer comprises a touch-sensitive surface activated by the finger or the hand of a user or any other actuation means and makes it possible to control an item of equipment or a system through a graphic interface presented by the screen. There are a large number of possible uses. In particular, there are the aeronautical applications in which a pilot can thus monitor and control all the functions displayed by the avionics system of the aircraft.
Such interfaces have to present feedback to the user, for example the change of state of a button, so that the operator can see the transition from non-activated to activated, and thus confirm his interaction. The touch-sensitive surface mode of interaction with a user is not very collaborative and errors can occur, associated with the doubt concerning the activation or non-activation of the command, for example in the case of blind commands where the pilot must first of all carry out his main task by looking outside. For this, the display devices may involve the auditory sense of the user by generating a sound to provide the feedback.
However, in a noisy environment, notably an aircraft cockpit, it is not always easy to perceive this feedback.
Also known are haptic systems with force feedback which make it possible to increase the reality of the interaction, by providing physiological feedback concerning the state of the activated object. According to the state of the art generally found, this feedback is triggered solely on the condition of presence of the activation means, generally the finger. In reality, when a control element of a system is activated, such as, for example, a pushbutton or a potentiometer, the latter reacts to the press action, because it has a reaction function, which may be its stiffness, its travel, its viscosity, its triggering threshold. For example, a pushbutton has a pre-travel, its depression law depends on its stiffness and on the force applied by the activation means. However, beyond a certain value, the reaction force of the pushbutton decreases abruptly, and it is at this precise instant that activation occurs (and only if the force has reached this threshold). This law is reversible when the button is released. This property is called the haptic reaction function. This function involves physical touch, also called kinesthesics, that is to say, related to the nervous and muscular system of the operator. Similarly, a surface state, a texture, may be characterized by a haptic reaction function, and in this case, involve sensory receptors located at the ends of the nerve terminations of the skin, called pacinian receptors.
There are haptic systems that use a touch-sensitive surface set in vibratory motion by means of actuators. An actuator is an electromechanical system which makes it possible to transform electrical energy into mechanical energy more often than not in the form of a motion of a mechanical part. It is generally driven by a controller linked to a computer device.
These actuators are of different types:
Eccentric rotor mass (ERM): This is an eccentric mass which creates radial inertia forces. All of the system vibrates, and because of this, this device applies only to roaming instruments. In practice, for a fixed item of equipment on a rigid support, the effect will be zero, or worse, if the support is flexible, it might enter into resonance.
Seismic resonant mass (LRM): A mass suspended by a spring is set into resonance by an electromagnetic or electrostatic device. This principle suffers from the same drawback as the first.
Electromagnetic: The actuator consists of a frame that can be distorted by the displacement of a core within a coil. The lateral displacement produced is retransmitted to the screen by virtue of an armature. Such a device is, for example described in the Patent WO2006124873A1.
Piezo bimorph: Two types of actuators are used: vibrating plate (“piezo beam”) and blister discs (“piezo disc”). These actuators use the shear forces induced by the piezoceramic covering one or two faces of the actuator. An example of such a device is given in the Patent US2008122315A1.
Electrostatic: By capacitive effect, two parallel layers covering the screen are drawn together or pushed apart to stimulate the interacting object.
With shape memory: Some materials revert to a particular shape when they are subjected to a certain temperature.
These generalities will be better understood by referring to “Tactile-Feedback Solution for an Enhanced User Experience, M. Levin and A. Woo, Information Display October 2009 Vol. 25 No. 10”
Not all these systems are suited to reproducing kinesthesic effects, and particularly on large screens, producing significant inertial forces under dynamic stress. For example, in the Patent Application US2007080951A1, piezo bimorph actuators are arranged directly behind a touch-sensitive surface between a support frame and the touch-sensitive surfaces, but they deliver only forces of the order of a Newton, which is insufficient to oppose the force of an operator. In addition, the large touch-sensitive surface, that is to say, the surfaces that have a diagonal that is at least equal to 15 inches, have low resonant frequencies making it difficult to generate vibro-tactile effects to the frequencies sensitive to the pacinian receptors which lie within a frequency band of the order of 150 to 250 Hz. In addition, the actuators cited and currently used produce forces normal to the screen that are very low, well below what an operator can produce. It would be possible to consider multiplying the number of actuators, but this would increase the price, the consumption, the bulk and the weight of the system.
Piezoceramics are known which are elongated according to the voltage applied. However, this elongation is of the order of 0.1%. Thus, to obtain 0.5 mm of travel, an actuator 50 cm long would be needed, which is not feasible on a flat screen.
Also known is the International Patent Application WO2009/088707 describing a display device comprising a touch-sensitive surface, a frame on which the touch-sensitive surface rests and actuators assembled with rails producing motions parallel to the plane of the touch-sensitive surface. The rails extending longitudinally along the frame are mechanically coupled with the frame and the fixed support of the display device. The rails make it possible to reduce the number of actuators needed in such a device. The actuators used to implement the haptic device are not suited to use in large size screens.
Some devices laterally displace the touch-sensitive surface, so as to benefit from a greater rigidity, but in this case, the reproduction of kinesthesic effects is no longer possible, since the force applied by the operator is normal to the screen, and the device cannot oppose it.
Another problem is the rigidity of the screen, the latter having a tendency to flex at the centre. Because of this, under a lateral excitation, it will have a tendency to resonate at a lower frequency and therefore to damp the transmitted effects.